The "Star Wars" project wants to use a radically new propulsion system, a nuclear reactor, to boost anti-missile weapons into Earth orbit, according to an arms control group that opposes the idea.

The nuclear rocket, dubbed Project Timberwind, could be used to put "directed energy" weapons in orbit to attack incoming missiles with particle or radiation beams, Steven Aftergood of the Federation of American Scientists said Tuesday.The short-notice launches of such weapons would need rockets of enormous power. A nuclear rocket - carrying no crew that would need heavy shielding - could probably triple the payloads of conventional rockets, Aftergood estimated.

"Any existing system could really be souped up," he said in a telephone interview.

But, he said, a nuclear rocket "would be a gross violation of safety standards the United States has endorsed" - that space reactors ought not to be started until a stable orbit or a path into deep space has been reached.

Aftergood said he believed political objections would kill the program.

The federation was formed by some nuclear scientists after World War II to advocate arms control. Aftergood, a senior research analyst for the group, has frequently argued against placing nuclear devices in space.

Project Timberwind "has gone beyond paper and pencil," he said.

"Some fuel has been fabricated. Some tests on the fuel have been made. A test site, at Saddle Mountain . . . in the middle of the Nevada Test Site has been picked. A flight test has been proposed" for the reactor on an over-water course around Antarctica, he said.